Wafer-level packaging is an arrangement in which individual dies are pre-packaged with a redistribution layer before cutting the wafers to singulate the dies. This enables the use of sophisticated semiconductor processing techniques to form smaller arrays of electrical interconnects such as solder balls. Additionally, wafer-level packaging is an efficient process that simultaneously packages a plurality of dies, thereby reducing costs and increasing throughput.
Conventional wafer-level packaging includes electrical interconnects (referred to hereinafter as “solder balls” although other types of interconnects may also be used) between a semiconductor device and a substrate on which it is mounted that are generally quite small and thus may be damaged. Thermal cycling and different coefficients of thermal expansion of the structures connected to the solder balls further stress these connections and can eventually lead to failure.
One method of increasing the bonding strength between the solder balls and a substrate involves forming a layer of material, which may comprise, for example, an epoxy-like molding compound, around an array of solder balls to at least partially encapsulate the solder balls while leaving a portion of each solder ball exposed for subsequent connection to another electrical contact. FIG. 10 shows such a conventional approach to solder ball encapsulation in which a substrate 1000 has a plurality of solder balls 1002 connected thereto which are partially covered with an encapsulant 1004 in a manner that leaves end portions 1006 of the solder balls 1002 projecting from the encapsulant.
While this approach strengthens the connection between the solder balls and the semiconductor device to which they are mounted under some conditions, it has been found that under other conditions it may not provide the same degree of strengthening. First, thermal expansion of the molding material may stress the connections between the solder balls and the substrate. Also, it has been found that during reflow processes in which the solder balls are heated to allow for bonding, the solder balls do not collapse the same way as they do when they are not encapsulated. The type of solder ball collapse that occurs in the presence of the encapsulant sometimes results in weakened portions of the reflowed solder balls and/or creates stress concentration points which may be more prone to failure. It would therefore be desirable to obtain the benefits of conventional solder ball encapsulation while avoiding the aforementioned issues.